The term cytochrome P-450 represents a family of heme-containing enzymes that collectively catalyze the oxidation of virtually an infinite number of endogenous and exogenous compounds. Each of the enzymes has its own substrate specificity, but they possess the unusual characteristic of forming several different metabolites either by parallel reactions or in sequence. Although the formation of several different products implies the existence of different orientations of the substrate within the active site of the enzyme, the mechanisms by which these different orientations are achieved may be markedly diverse. Studying the isotope effects on the metabolism of different substrates is a highly useful approach for elucidating various features of the enzymatic cycles. To this end, we have derived enzyme kinetic equations for several plausible models by which a cytochrome P-450 enzyme may form several metabolites from a single enzyme. The equations illustrate the usefulness of estimating isotope effects not only on isotope-sensitive pathways, but also on isotope-insensitive pathways and on total metabolism in differentiating between different mechanisms of product formation.